1. Field of the Invention
The present invention relates to an apparatus for measuring the amount of liquid in a tank and, more particularly, to an apparatus for measuring the weight, level and/or volume of a liquid in a sealed container.
2. General Background
The prior art suggests a number of methods and apparatus for measuring the liquid level in a tank. Some utilize ultrasonics while others use a float system in combination with the magnetostrictive principle. Other apparatus rely on sensor arrays to determine the mass of the liquid in a container then calculating the volume based on the mass, specific gravity of the liquid and adjusting for temperature variations. All such devices and systems have one thing in common, they all have various degrees of error. Each system strives to reduce the error, and in doing so, the systems become more complicated. One such system is disclosed by Zfira in U.S. Pat. No. 5,157,968. Zfira discloses a system for measuring the specific gravity, weight, level and/or volume of a liquid in a container. However, Zfira employs a dual system, one for measuring specific gravity where the liquid may change from time to time and the second for measuring the mass of the liquid thus determining the quantity of liquid in the container. The system requires two load cells, two body weights and two transmitters to send data to a computer monitoring system which, in effect, produces two input signals, thereby, inducing a second error factor. Any system which relies on multiple sensors or inputs compounds its potential for error. The Zfira patent does not disclose a means for operating under pressure. The known systems are usually relatively complicated and are limited as to the types of liquid measured without using multiple sensors. Such systems do not address the need for a simple system having a single input for measuring a known type of material, contained in a vessel, designed for storing a specific volatile liquid. In some such cases, it is essential that the apparatus be capable of operating while under pressure and be calibrated without opening the enclosure or otherwise disturbing the integrity of the vessel.